Abstract Cooperative multi-hop transmission in wir

Cooperativemulti-hoptransmissioninwirelessnetworksPatrickHerhold*,ErnestoZimmermann,GerhardFettweisVodafoneChairMobileCommunicationsSystems,TechnischeUniversita¨tDresden,D-01062Dresden,GermanyAvailableonline1June2005AbstractWeconsidervariousrelayingstrategiesforwirelessnetworksbycomparativelyexaminingdirecttransmission,con-ventionalrelaying,andthenovelconceptsofcooperativerelaying.Thelatterbuildontwoinherentbenefitsofrelayingsystems:thespatialdiversityofferedbytherelaychannel,andtheabilitytoexploitthebroadcastnatureofthewirelessmedium.Studiedcooperativeprotocolsincludeadaptivedecode-and-forwardschemesasasimpleextensionofconven-tionalstore-and-forwardrelayingsystems,andmorecomplexdecode-and-reencodingschemesthatrealizedistributedcodingstrategies.Weprovideaunifyinganalysisforthetractabletwo-hopcase,beforeextendingtheconsiderationtomulti-hopscenarios.Theanalysisisconductedfromtheperspectiveofcommunicationoverfadingchannelsunderlim-itedbandwidth,energy,andend-to-enddelay;mainparametersincludepropagationloss,networkgeometry,andtar-getedend-to-endspectralefficiency.Mainresultsindicatethat(i)cooperativerelayingprovidesattractivebenefitsforwirelesssystemswhenevertemporalandfrequencydiversityarescarceornotexploited,(ii)usingjusttwohopsisrea-sonableformanypracticalscenarios,and(iii)theadvantagesofthestudiedrelayingschemesdecreaseforhigherdesiredend-to-endspectralefficiency.
2005ElsevierB.V.Allrightsreserved.Keywords:Multi-hopcommunications;Relaying;Usercooperation1.IntroductionGiventheconstraintsimposedbylinkbudgetestimatesforfuturegenerationsofinfrastructurebasednetworks,relayingemergesasaviableoptionforchallengingthetradeoffbetweentransmissionrangeandend-to-enddatarate.Essentially,relaysallowforreducingtheend-to-endpathlossbetweenaninfor-mationsourceanditsdestination.Eachrelayinarelaychaintherebyusuallyreliessolelyontheinforma-tionsentbyitsimmediatepredecessor,andthedestinationsimplylistenstothelastrelayinthischain.Werefertothisasconventionalrelayingasitisknownfromadhocnetworking.1389-1286/$-seefrontmatter
2005ElsevierB.V.Allrightsreserved.doi:10.1016/j.comnet.2005.05.009*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](P.Herhold).ComputerNetworks49(2005)299–324
transmissionsatthedestination,thespatialdiversityofrelayingsystemscanbeexploited.Moregenerally,usefulsideinformationistakenintoaccountthatisunnecessarilydiscardedinconventionalrelaying.Inadifferentcontext,thepromisingconceptsofspatialdiversityhaverecentlyleadtointenseresearchintheareaofmulti-antennasystems.Whilesystemsthatusethespatialdiversityofferedbyantennaarraysareattractiveduetotheirsimplicity,theirsuccessfuluserequirestheintegrationofmultipleantennaelementsatterminals,and,totakefullbenefit,uncorrelatedchannelsfromeachofthem.Inscenarioswheretheseconditionscannotbemet,cooperativerelayingprovidesanalternativeby‘‘distributing’’theantennasamongterminals.Cooperativerelayinghasalsobecomeknownasusercooperationdiversity,virtualantennaarrays,codedcooperation,anddistributedcoding.1.1.ChallengesInsmalldevices,theinabilityofRFhardwaretoperformsimultaneousreceptionandtransmissionatthesamefrequencycallsforassigningorthogonalchannelsforthereceivingandtransmittingpaths.Thisso-called‘‘orthogonalityconstraint’’incursasubdivisionofavailableresourcesintimeand/orfrequency.Inthispaperweassumethatthisdivisionisdoneinthetimedomain,whichallowsfordesigningcausal,adap-tivetransmissionconcepts.Asadirectconsequenceoftheorthogonalityconstraint,thetransmissionrateontheindividualphasesneedstobeincreased.Considerasystemwithlimitedbandwidthandend-to-enddelaythattransmitsfromsourcetodestinationatspectralefficiencyR.SubdividingthistransmissionintokhopsimpliesthateachofthehopsbeoperatedatratekRifthesameend-to-endefficiencyistobeachieved.Otherchallengesincludepossiblyincreasedinterferencethatresultfromrepeatedre-emissionsinrelaynetworks,protocolcomplexityrelatedtoroutingandscheduling,andsecurityissuesthatresultfromrelay-ingdataviaanotheruser
sterminal.Interestingly,itturnsoutthatmostofthesechallengesarenotrelatedtousercooperationassuch,buttorelayingingeneral.1.2.ClassificationRelayprotocolscanbeclassifiedaccordingtotheirforwardingstrategy:1.Amplify-and-forward:Relaysactasanalogrepeatersbyretransmittinganamplifiedversionoftheirreceivedsignals.Thenoisefloorisincreased.2.Decode-and-Forward:Relaysattempttodecode,regenerateandretransmitanexactcopyoftheoriginalsignal,potentiallypropagatingdecodingerrors.3.Decode-and-Reencode:Relaysattempttodecodeandconstructcodewordsthataredifferentfromthereceivedcodewords,therebyprovidingincrementalredundancytoareceiverthatassessestheoriginalandthere-encodedsignals.Again,thereistheproblemoferrorpropagation.Asynonymoustermforamplify-and-forwardrelayingis‘‘non-regenerative’’relaying;thedecodingprotocolsarecorrespondinglyalsoreferredtoas‘‘regenerative’’relaying.Further,therearedifferentprot